In an electrophotographic printing machine, an image to be printed is produced on photosensitive material. The image may be comprised of text, line drawings, photographs, bar codes, or any combination of those data types. Light for exposing the electrically-charged photosensitive material may originate from a laser source, an LED source or any other suitable light source. Light modulating means are used to change light power levels or the time duration of exposure in order to produce variations in the charge level on the photosensitive material to reflect the charge level of voltage needed to reproduce the desired image.
After the desired image is reproduced on the photosensitive material, it is passed into a developer and then transferred to image-receiving material, for example, paper. The developing material is then fused into the paper for production of a completed print.
In order to continually produce a succession of satisfactory prints, it is desirable to adjust and readjust the parameters of the electrophotographic process to their optimum levels. For example, a specific charge level must be chosen to charge the photoconductor adequately prior to exposure. Since many types of photosensitive material (photoconductor) cannot be discharged to zero volts through exposure, a specific charge level, which will represent a satisfactory fully discharged image level, should be chosen. Voltage levels for the developer should be set together with appropriate levels for all light producing mediums. Once all these parameters are adjusted, they need to be continually monitored as the printer is used since print quality may degrade significantly as the photoconductor ages, or as heat, humidity and other environmental conditions affect the machine.
One of the problems facing machine designers is setting process parameters to a level that protects the boldness of printed material. That is, under certain circumstances, operating parameters which are optimized for producing solid black on a white background, may produce printing which is too bold (i.e., the white "hole" in a P may be obliterated), or printing which is not bold enough (i.e., narrow lines become invisible).
It is an object of this invention to achieve control over print boldness which is independent of the setting of machine parameters so that those parameters can be adjusted to compensate for the aging of photoconductor and other environmental conditions within a machine without affecting the boldness quality which the user desires.
Another object is to provide a system in which the user can adjust print boldness to a desired level (that is, through a range of boldness levels), and that adjustment is obtained without affecting the settings of machine parameters.
To achieve the objects of the invention, the inventors have recognized that photosensitive material is exposed by producing a succession of light spots, i.e., picture elements (PELS), which can vary in size. It is known that large spots are needed to prevent scan lines between spots, while small spots are needed to print fine details. Others have optimized spot size. This invention deals with setting and retaining the placement of image edge location by concentrating on retaining print fidelity. Fidelity refers to a true spatial reproduction of the printed PEL pattern for a given addressability and is most concerned with controlling the location of the printed edge. Edge location refers to all implied spatial consequences such as boldness, stroke width, dot size, and hole size. Fidelity control, as described in this invention, provides an independent control of print fidelity allowing improvements in print quality with greater latitude in the optimization of machine parameters. Edge PELS in this invention are defined as the discharged PELS directly adjacent to black edges. If a charged area development (CAD) process is in use, the edge PELS are those discharged PELS directly adjacent to the charged black PELS. In a discharged area development (DAD) process, the edge PEL is that discharged black PEL directly adjacent to a charged white PEL. This invention involves control of the exposure level of the edge PEL in order to produce control over the printed edge location.
U.S. Pat. No. 4,460,909 relates to enhancing the apparent resolution of an electrophotographic printer by writing grey PELS along the edges of slanted lines in order the smooth the digitization of the slanted line which might otherwise be visible.
U.S Pat. No 4,544,264 relates to broadening fine lines by adding small black areas to each edge of the fine line in order to broaden it, or in another dimension, broadening the fine lines by placing grey PELS next to black PELS.
U.S. Pat. No. 4,625,222 relates to the above two patents and provides means for improving print quality when the above two print enhancement techniques interact to degrade the resultant print.
U.S. Pat. No. 4,681,424 relates to providing environmental compensation, such as to modify the print data to broaden lines as needed.
U.S. Pat. Nos. 4,396,928 and 4,395,721 relate to systems for broadening scan lines so that adjacent scanning lines are partially superposed over each other.
U.S. Pat. No. 4,437,122 relates to a system for inspecting neighboring PELS in order to increase the density of the information elements to round off character edges and smooth diagonals.
U.S. Pat. No. 4,700,201 relates to a technique for altering the exposure of a PEL to control dot size in accordance with the value of neighboring PELS in order to achieve a smoother representation of half-tones.
U.S. Pat. No. 4,122,409 relates to control over laser intensity by controlling the current level supplied to the laser.
U.S. Pat. No. 4,476,474 relates to the use of multiple laser beams to control spot size.
U.S. Pat. No. 4,821,065 relates to a system for controlling laser power in accordance with measurement of surface potential.
An article, "Fidelity Control of Electrophotographic Print Process" by J. L. Crawford, SPSE/SPIE Symposium on Electronic Imaging, Jan. 20, 1989, describes the science of the instant invention.